The present invention relates to a synchronous motor and in particular to a synchronous motor for use, by being energized from a high-frequency electric power supply, to drive an extra-high speed rotary machine such as a grinder, a compressor, and a centrifugal separator or the like, and being one of the so-called permanent magnet type wherein exciting windings are omitted in its rotor and a permanent magnet is employed instead.
For a simple construction in that the magnetic poles of the rotor are constructed by the use of a permanent magnet instead of exciting windings, the synchronous motor of the permanent magnet type finds increasing use.
The rotor provided with the permanent magnet encounters little problem when it is used for the motor operating at a relatively low rotational speed, while when it is used for the motor operating at a high rotational speed it meets a problem that it is very difficult to design the rotor capable of tolerating the centrifugal force applied thereto when the motor is running at a high rotational speed, only depending upon the mechanical strength of the permanent magnet. This also results in the poor reliability of the machine.